To a blender, add avocados, roasted serrano chile, and lime juice. Blend until smooth (add water as needed to blend smoothly). Season to taste with salt and pepper.
Peppers on the grill. If I can remember, I throw a few peppers on the hottest part of grill while I’m making something else. That way I have some when I need them. Grill the whole peppers until the skin is blistered and black in parts, but not white (ash).
Roasting peppers on the stove. Turn a gas stove on HIGH, then position the pepper directly over the flames, turning with tongs until the pepper's skin is black and blistered.